The present invention relates to a polymer electrolyte fuel cell for use in portable power sources, power sources for portable devices, power sources for electric vehicles, domestic cogeneration systems and the like.
A fuel cell using a polymer electrolyte membrane electrochemically reacts a fuel gas containing hydrogen with an oxidant gas containing oxygen, such as air, to simultaneously generate electric power and heat. This fuel cell comprises a polymer electrolyte membrane for selectively transporting hydrogen ions, and a pair of electrodes, i.e. an anode and a cathode, formed respectively on both faces of the polymer electrolyte membrane. This is called an electrolyte membrane-electrode assembly (MEA). The electrode comprises: a catalyst layer mainly composed of a carbon powder carrying a platinum group metal catalyst and formed on each face of the polymer electrolyte membrane; and a gas diffusion layer formed on the outer surface of the catalyst layer and having both permeability and electronic conductivity.
In order to prevent a fuel gas and an oxidant gas (reactive gases) to be supplied from leaking out or prevent these two kinds of reactive gases from being mixed with each other, sealing members, such as gaskets, are disposed on the peripheries of the electrodes, with the polymer electrolyte membrane sandwiched therebetween. The sealing members are previously assembled integrally with the electrodes and the polymer electrolyte membrane, and this is called an electrolyte membrane-electrode-sealing member assembly (MESA).
Conductive separator plates are disposed on the outer sides of the MEA for mechanically fixing it and for electrically connecting adjacent MEA's with each other in series. Each of the separator plates has a gas flow channel for supplying reactive gases to the electrode and for carrying away a generated gas and an excessive gas. Although the gas flow channel may be provided separately from the separator plate, a general manner is to provide grooves on the surface of the separator plate to serve as a gas flow channel.
In order to supply reactive gases to these grooves, it is necessary to branch pipes for supplying reactive gases, according to the number of separator plates to be used, and to use jigs for connecting the end of each branch directly to the grooves on the separator plate. This jig is called a manifold, and a type of manifold that directly connects the pipes for supplying reactive gases to the grooves as mentioned above is called an external manifold. A type of manifold with a simpler structure than the external manifold is called an internal manifold. The internal manifold is configured such that through holes are formed in the separator plates having gas flow channels and the inlets and outlets of the gas flow channels are extended to these holes so as to supply reactive gases directly from the holes.
Since a fuel cell generates heat during operation, it is necessary to cool the cell with cooling water or the like in order to keep the cell in good temperature conditions. Normally, a cooling water flow channel is provided for every 1 to 3 unit cells. It is often the case that a cooling water flow channel is provided on the rear surface of the separator plate to serve as a cooling portion. These MEA's and the separator plates are placed one upon another to form a stack of 10 to 200 unit cells, and this stack is sandwiched by end plates, with a current collector plate and an insulating plate between the stack and each end plate, and then fixed with clamping bolts from both ends of the stack. In this manner, a polymer electrolyte fuel cell having a typical structure can be obtained.
A sealing member to be used in such a polymer electrolyte fuel cell as described above is required to have high dimensional accuracy, sufficient elasticity and sufficient fitting margin in order to seal reactive gases, while bringing the separator plate into contact with the electrode. As a typical sealing material, therefore, a seat-shaped gasket comprising a resin, rubber or the like, or an O-ring-shaped gasket comprising rubber, has been used.
Recently, for example, an attempt has been made to reduce load needed for sealing with gaskets for the purpose of simplifying constituent members and reducing the weight and cost thereof by reducing load for clamping a stack, as disclosed in Japanese Laid-Open Patent Publication No. Hei 11-233128 and Japanese Laid-Open Patent Publication No. 2002-141082. Further, another attempt has been made to make a cross section of a gasket triangular, semicircular or the like, instead of making it O-ring-shaped.
In the case of using a gasket having an O-ring-shaped cross section and a certain degree of cross sectional area, it has been attempted that the gasket is disposed on the separator-plate side. However, such a gasket has a problem of being inappropriate for reliably securing a sealing property since a large number of unit cells are stacked to be clamped in a stack.
When an O-ring-shaped gasket is used, sealing is conducted by clamping an electrolyte membrane onto separator plates with the gaskets. Therefore, sealing needs to be conducted in two locations: between an anode (fuel electrode) and the electrolyte membrane; and between a cathode (oxidant electrode) and the electrolyte membrane, and that is to say, both a gasket for sealing a fuel gas and a gasket for sealing an oxidant gas are required, raising a problem of enlarging portions needed to be sealed.
It is further necessary to provide grooves, into which the O-ring-shaped gaskets are placed, on the surface of the separator plate, thereby setting restrictions, such as a restriction on reduction in thickness of the separator plate in order to secure the groove dimension. This has brought about an increase in stack volume, an increase in cost, and complication of the form of the separator plate, causing a decreased yield in processing the separator plate. With the aim of solving these problems, it has been attempted to conduct sealing in a smaller space.
In building a stack, an MESA or MEA is disposed on the separator plate, and on this MEA, the separator plate, or the O-ring-shaped gasket and the separator plate, are disposed. This process is repeated to obtain a stack. In such disposition of the O-ring-shaped gasket or the separator plate on the MEA, a guide is used as a typical jib for assembly. However, since there are dimensional deviations among the respective members, and from the viewpoint of facilitating stacking of the electrodes and the O-ring-shaped gaskets or the separator plates, a clearance is needed between the O-ring-shaped gasket and the electrode. This clearance is aimed at securing good operability or production yield.
When this clearance is small, it tends to be difficult to reliably build a stack. For example, the O-ring-shaped gasket may be partially placed or stacked on the electrode to cause poor sealing. Further, with the O-ring-shaped gasket brought into contact with the electrode, excessive surface pressure is applied onto the electrode, which may result in damage of the electrolyte membrane or deterioration in durability, whereby cell performance may be degraded.
In the case where the clearance between the O-ring-shaped gasket and the electrode is made smaller, therefore, a yield may decrease and cost for components may increase unless dimensional accuracy of each component is improved. Especially when a molded separator plate is used, it is difficult to reduce the clearance between the O-ring-shaped gasket and the electrode since there is a limit to process accuracy of a guide to be used in building a stack, or the like. For this reason, after production of the separator plate by molding, a guiding portion has been added in a post-process, and this has generated additional cost.
On the other hand, in the case where the clearance between the O-ring-shaped gasket and the electrode is enlarged for the purpose of securing a property of building a stack, a reactive gas may flow into the clearance, and it is thereby possible that the reactive gas may be prevented from flowing in a gas flow channel of the separator plate. Further, when clearances of the respective unit cells vary, attributed to deviations in stacking the MEA's and O-ring-shaped gaskets, pressure losses among the unit cells also vary. Because a reactive gas flows in each unit cell in an amount according to pressure loss of each unit cell, flow rates of the reactive gas vary. This causes variations in cell performance among the unit cells, having harmful effects such as lowering of power generation voltage, deterioration in durability and deterioration in safety during low output power operation. These problems occur more significantly on the fuel-gas side where a utilization rate of a reactive gas is relatively high.
Moreover, when a flat gasket is used, although the volume occupied by the gasket can be reduced, the aforesaid problems regarding building of a stack and a clearance exist as in the case of using the O-ring-shaped gasket. Further, excessive clamping force is required for securing surface pressure needed for sealing. It is therefore difficult to reduce weight and cost of members for clamping a stack, and compact those members.